will not start

I have a 1991 Cadillac coupe deville. i have 168000 miles on it. it has a 4.9 liter engine. all of a sudden the car has no power. when i turn the key to the start position nothing happens. i have tested the battery, battery is good. i have traced all wiring, everything looks good. i have checked all the fuses in the car. everything is fine. just cant figure out what to check next. thanks billy